Fossil Ecosystems of North America: Mistaken Point

The CAMBRIAN EXPLOSION

"Although multicellular animals had only appeared at the very end of the Precambrian,
their evolution at the beginning of the Cambrian was so rapid that this event is known as the Cambrian Explosion"

In the previous chapter we have studied the Ediacaran Fauna at Mistaken Point, the oldest multicellular animals appearing in the fossil record; in this chapter we will move to British Columbia, Canada, where the Burgess Shale Lagerstätten shows evidence for a "boom" in animal life.
Is the boom real (there are actually several new animals whose fossils show up in the record) or is it an artifact (we only see several new species because many of them developed shells, that are much easier to preserve and hence have a higher chance to be fossilized)?

You will NOT be quizzed on the following three links, so you do NOT HAVE TO check them out if you do not want to; if you are curious though, here is some more info on these wonderful Lagerstätten:

From the Smithsonian National Museum of Natural History in Washington D.C.
From the University of California, Berkeley, California
From the Burgess Shale Foundation in British Columbia, Canada

Crossing into Canada at the end of I-15, in Montana.
The I-15 is the fastest road from Santa Monica to the Burgess Shale in British Columbia.

part 1 - BACKGROUND: THE CAMBRIAN EXPLOSION

What is the "Cambrian Explosion"?
In a mere 10 million years at the beginning of the Cambrian almost every animal phylum and body plan known today (along with some bizarre forms which soon disappeared) shows up in the fossil record.
What does this sequence of evolutionary events suggest?
What kind of perplexing questions arise from such a sudden appearance of this diverse fauna?

It seems like the Precambrian Ediacaran Fauna (at Mistaken Point and the Ediacara Hills) may not represent completely the ancestors of the Early Cambrian animals. The first pulse of Cambrian life occurs with the sudden appearance of many small shelly fossils. Shells, so common today, did not exist before the base of the Cambrian. Since shells are made of minerals, mostly calcium carbonate, they can resist decomposition. These animals with shells, maybe they descend from Precambrian creatures without shells, which cannot be found, or they represent simple spines or spikes that were part of soft-bodied animals. Either way, these Small Shelly Fossils do not show the variety of body plans that is evident later on during the Cambrian.

Soon after the Small Shelly Fossils, our "boom" in animal life occurred. Maybe because of improved environmental conditions and the opening of ecological niches, or as a consequence of a "race to arms" between these creatures, or other factors still unknown, accelerated diversification occurred.

Much of our knowledge about this "boom" comes for the Burgess Shale Lagerstätten. In the Burgess Shale, 85% of the fossil genera are completely soft-bodied, and as such they are missing from any other Cambrian fossil assemblage. This means that, if it were not for the Burgess Shale, we would miss 85% of the fossils of the time, and hence we would have a very different vision of how animals came to be on Earth.

part 2 - HISTORY OF DISCOVERY OF THE BURGESS SHALE

You should read this paragraph and know who the main characters are, from the discoverer, Charles D. Walcott, to Raymond, Simonetta, Briggs and Conway Morris. You should know where most of the fossils that Walcott found are preserved.

part 3 - STRATIGRAPHIC SETTING AND TAPHONOMY OF THE BURGESS SHALE

Again, you do not need to know names of Formations from this section. You need to read the whole paragraph and understand the settings of the rocks where the fossils were preserved (at the foot of a near-vertical submarine cliff)

What is the conventional interpretation of the sedimentary environment?
What were the conditions in the water offshore from the cliff?
Were the Burgess animals preserved in the area they were living? Yes or no, and why?
What is the accepted theory of the deposition of the Burgess animals?
What are two of the common prerequisites for soft-tissue preservation?
Why do the book authors sustain that there must be at least a third cause for preservation?
Clay minerals in the mud turned in time into calcium aluminosilicates similar to micas. Why is this important in the Burgess Shale?

part 4 - DESCRIPTION OF THE BURGESS SHALE BIOTA

Out of these fossils, you need to know about the following: Hallucigenia, Anomalocaris, Opabinia, and, above all, Pikaia. Pikaia shows a stiff rod, a primitive "backbone" that makes it the ancestors of all vertebrates, including of course us.

part 5 - PALEOECOLOGY OF THE BURGESS SHALE

Know the environment in which the Burgess Shale organisms lived
Know the distinction between plankton, nekton, and benthos
You do not need to know the percentages of what is what among these creatures, but you need to understand the first ever recorded predator roles of Anomalocaris and Opabinia
Know about Pikaia

part 6 - COMPARISON OF THE BURGESS SHALE WITH OTHER CAMBRIAN BIOTAS

Be able to outline the main differences that exist between the Burgess Shale biota and those form Sirius Passet in Greenland, and Chengxiang, China

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